Hybrid combustion/electric or electric vehicles include, in particular, high-power batteries. Such batteries are used to drive an AC electric motor by way of an inverter. The voltage levels required for such motors reach several hundred Volts, typically of the order of 400 Volts. Such batteries also comprise a high capacity so as to further the vehicle's range in electric mode.
To obtain high powers and capacities, several groups of accumulators are placed in series. To facilitate the manufacture and the handling of the battery, the accumulators are generally grouped together in several modules connected in series. The number of stages in a module and the number of accumulators in parallel in each stage vary as a function of the desired voltage, current and capacity of the battery. The electrochemical accumulators used for such vehicles are generally of the ion lithium type for their capacity to store significant energy with a contained weight and volume. Battery technologies of Lithium ion iron phosphate LiFePO4 type form the subject of significant developments on account of a high intrinsic level of safety, to the detriment of a somewhat restricted energy storage density. An electrochemical accumulator customarily has a nominal voltage of the following order of magnitude:
3.3 V for a lithium-ion iron phosphate technology, LiFePO4,
4.2 V for a technology of cobalt oxide based lithium-ion type.
The document “Detection of Electric Arcs in 42-Volt Automotive Systems” describes various methods for detecting arcs. This document describes in particular a method for detecting arcs by ultrasound. In practice, this document describes the fact that acoustic sensors have to be distributed at various places in the vehicle to detect arcs for various localities of the electrical harness. Acoustic sensors are also envisaged for detecting arcs for interconnections of an electrical source, the source and the interconnections being protected in a housing, therefore forming a separation with the exterior. This document concludes as to the impossibility at this juncture of obtaining a locating solution based solely on acoustic sensors, on account of sensitivity to errors. This document does not afford any solution to the detection of electric arcs of the interconnections of the electrical source.
Document U.S. Pat. No. 4,442,700 describes a device for measuring the humidity inside a battery. This document describes in particular a battery furnished with a protective housing and electrical energy storage cells disposed in the housing and connected electrically in series by interconnection elements. Bus bars serve to connect plates of one and the same cell. Liquid electrolyte bathes the plates inside the housing. Acoustic sensors are disposed against the wall outside the housing and are thus protected against the corrosion of the electrolyte. On the basis of the measurement of the sensors, the document proposes to determine the humidity and the state of charge of the battery.
Document JP2010-101706 describes a battery accommodated in a protective housing. The battery is furnished with an ultrasound transducer fixed outside the battery and used to emit an ultrasound signal in the battery and to measure the ultrasound response of the battery. As a function of the response, a circuit determines the capacity of this battery. This determination of capacity is based on the diffusion of compounds in the electrolyte with battery wear, which modifies the ultrasound response.
Given the quantities of energy stored in power batteries intended for the traction of automotive vehicles, failures of such batteries may have considerable consequences.
One type of potential failure is the occurrence of electric arcs at the level of the series connection between electrochemical accumulators. Electric arcs usually result from wear, corrosion, vibrations, or an accident related impact. With a DC voltage source, such as a battery, the electric arcs remain struck as long as current is drawn by an electric load.
Electric arcs appear when a space is created between two electrical conductors in which current flows and electrical conduction continues across this space. The arc comprises two arc foot zones extending from the two respective electrical conductors. These arc foot zones generally exhibit a length of between 5 and 10 μm and a temperature of the order of the melting temperature of the two electrical conductors. This temperature is, for example, on the order of 1000° C. for copper electrical conductors. These arc foot zones are separated by a plasma exhibiting a temperature of the order of 10,000° C. for example. This temperature depends on the thermal equilibrium between the energy intake related to the current and the cooling by radiation essentially. The more significant the current and/or the shorter the arc, the higher is this temperature.
Electric arcs induce extremely high heating that may result in the melting of a connecting joint between the accumulators and surrounding materials.
Thus, if such electric arcs are not detected sufficiently early, their consequence may be the starting of a fire. Consequently, a need exists to detect occurrence of electric arcs. The detection of an electric arc turns out to be relatively arduous in a battery, the electrochemical cells generally being accommodated permanently in protective housings. The battery exhibits a complex structure with numerous mechanically isolated zones that are difficult to access. Moreover, a battery comprises a large number of interconnections that are liable to generate an electric arc.
A generic procedure for detecting arcs is based on a measurement of current and voltage. The occurrence of an electric arc induces disturbances in the voltage and the current at the terminals of the battery. By appropriate signal processing of these measurements, it is possible to detect the occurrence of an electric arc in certain applications. This solution is, however, inapplicable for batteries, since the electrochemical accumulators exhibit a very low impedance that attenuates the voltage signature of electric arcs. To alleviate this problem of detection, a large number of sensors ought to be used. This would induce an inappropriate cost for the detector.
Another generic procedure for detecting arcs relies on the measurement of optical radiation. In the presence of an electric arc, a very particular optical radiation is emitted. This radiation comprises the superposition of a continuous spectrum and of a discontinuous spectrum. The continuous spectrum is emitted during collisions of the electrons with the ions or atoms of the plasma. The discontinuous spectrum corresponds to photons, of well-determined frequencies, emitted by an atom, an ion, or a molecule that passes from one energy level to a lower energy level. This solution is, however, inapplicable for batteries, since the radiation sensor would be incapable of detecting the radiation of certain connections occluded by a protective housing of complex shape or disposed in the core of a module and occluded by electrochemical accumulators.
Another procedure for detecting arcs is used in photovoltaic panels. This procedure is based on the measurement of the electromagnetic field and the identification of a specific signature. Such detection induces a significant number of false alarms especially when the surrounding electromagnetic noise is significant. Such a solution is therefore unsuitable for automobile applications, subject to significant spurious glitches in the radiofrequency domain, in particular on account of the electric motorization. Moreover, such detection is greatly affected by any screen to the propagation of electromagnetic waves, the metallic elements such as a battery case or the chassis of the vehicle forming such screens. Furthermore, such detection exhibits a relatively long response time.
Another procedure for detecting electric arcs is used for overhead electric lines. Accordingly, use is made of a detector furnished with an acoustic sensor exhibiting a resonant frequency of 40 kHz. An electric arc indeed generally appears through the emission of an acoustic wave whose spectrum includes such a frequency. The acoustic sensor therefore measures a very specific value of the ultrasound disturbances in proximity to electric lines. The acoustic sensor exhibits very strong directivity. The signal provided by the acoustic sensor is modulated toward frequencies of the audible spectrum. The audible signal is provided to an operator by way of earphones. The operator determines the presence of electric arcs through the occurrence of an audible sound in the earphones. Such a detector is not applicable to a battery since the electrochemical accumulators are separated from the operator by the protective housing.
Thus, no procedure for detecting electric arcs in a power electrochemical accumulator battery is satisfactory for industrial application.